1. Field of the Invention
The present invention relates to a temperature-dependent current sensor circuit and a substantially constant intensity light source and corresponding method using this sensor circuit.
2. Brief Description of the Prior Art
Insertion of an integrated power factor controller circuit such as controller MC33262 from MOTOROLA in an electric power supply system enables easy and efficient control of the power factor and the level of current harmonics.
To obtain a power factor equal to unity, controller MC33262 draws current from the ac source in proportion to the sinusoidal voltage. This concept automatically causes the current waveform to be sinusoidal and in phase with the voltage waveform.
Also, operation of power factor controller MC33262 requires that the output supply voltage be higher than the peak amplitude of the input sinusoidal voltage in order to draw current from the ac source throughout every cycle of the sinusoid. Accordingly, the output supply voltage must have an amplitude higher than the peak amplitude of the sinusoidal voltage of the ac source.
In certain circumstances, an output supply voltage with an amplitude lower than the peak amplitude of the input ac voltage is required. In such cases, power factor controller MC33262 is used as a power-factor-correcting pre-converter; a second power converter is also required to reduce the level of the supply voltage to the desired amplitude.
Necessarily, providing a second power converter involves additional costs and requires more space.
Furthermore, the voltage/current characteristic of a light-emitting diode is sensitive to temperature causing the current through a light-emitting diode to change very rapidly and non-linearly with the voltage across the light-emitting diode.
For example, for a given type of light-emitting diode widely used in the fabrication of traffic signal lights, a constant voltage of 1.8 volts will produce in the light-emitting diode a current of about 7.5 mA at a temperature of xe2x88x9225xc2x0 C., a current of about 20.5 mA at a temperature of +25xc2x0 C., and a current of about 30 mA at a temperature of +60xc2x0 C. The magnitude of the current through the light-emitting diode at a temperature of +60xc2x0 C. is therefore, for a constant voltage of 1.8 volt, about 1.6 times higher than the magnitude of the current at a temperature of +25xc2x0 C. Voltage feedback control would therefore be very detrimental to the service life of such a light-emitting diode.
Since voltage feedback control of the supply of a light-emitting diode is not desirable, current feedback control is required to ensure durability of the light-emitting diode.
Also, a fixed LED output current presents the following drawbacks:
at higher temperature the output LED power decrease; and
at lower temperature the output LED power increases.
An object of the present invention is therefore to eliminate the above discussed drawbacks of the prior art.
Another object of the present invention is to regulate the output power, hence the light intensity, of a non-linear light-emitting load.
More specifically, in accordance with the present invention, there is provided a sensor circuit for detecting a current supplied to a non-linear load and for producing a current reading dependent on a condition of operation of the non-linear load. The sensor circuit comprises first and second serially interconnected resistors also connected in series with the non-linear load, and a variable impedance connected in parallel with one of the first and second resistors, the impedance varying with the condition of operation of the non-linear load. At least a portion of the current through the non-linear load flows through the sensor circuit to enable the first and second serially interconnected resistors and the variable impedance to produce a variable voltage signal representative of the current through the non-linear load and dependent on the condition of operation. In a preferred embodiment of the invention, the non-linear load is a light-emitting diode (LED) or a plurality of LEDs, and the condition of operation of the LED is temperature.
The invention described above therefore procures the advantage of providing a current-representative signal that may be used for feedback control of a non-linear load. Current feedback control is difficult with current sensor circuits which do not provide an output that varies with the condition of operation of the non-linear load. The invention described herein provides this feature in a simple low-cost circuit.
The present invention also relates to a substantially constant intensity light source comprising:
a) a non-linear light-emitting load;
b) a controllable dc voltage and current source for supplying the non-linear light-emitting load with dc voltage and current;
c) a current sensor circuit connected in series with the non-linear light-emitting load and the controllable dc voltage and current source, the current sensor circuit having an impedance varying with a condition of operation of the light-emitting load and being supplied with at least a portion of the current through the non-linear light-emitting load, whereby the variable impedance produces a variable current-representative signal; and
d) a voltage and current control feedback circuit connected between the current sensor circuit and said controllable dc voltage and current source for controlling the dc voltage and current source in relation to the variable current-representative signal to thereby adjust the dc voltage and current to amplitudes that keep the light intensity produced by the light source substantially constant.
Further in accordance with the present invention, there is provided a substantially constant intensity light source comprising:
a) a controllable dc voltage and current source having first and second terminals;
b) a non-linear light-emitting load connected between the first and second terminals and supplied with dc voltage and current from the controllable dc voltage and current source;
c) a current sensor circuit connected in series with the non-linear light-emitting load between the first and second terminals, the current sensor circuit having an impedance varying with a condition of operation of the light-emitting load and being supplied with at least a portion of the current through the non-linear light-emitting load, whereby the variable impedance produces a variable current-representative signal, and
d) a voltage and current control feedback circuit connected between the current sensor circuit and the controllable dc voltage and current source and through which the dc voltage and current source is controlled in relation to the variable current-representative signal to adjust the do voltage and current to amplitudes that keep the light intensity produced by the light source substantially constant.
The present invention still further relates to a method for keeping the intensity of a light source substantially constant, comprising:
a) supplying from a controllable dc voltage and current source a dc voltage and current to a non-linear light-emitting load:
b) supplying at least a portion of the current through the non-linear light-emitting load to a current sensor circuit having an impedance varying with a condition of operation of the light-emitting load, whereby the variable impedance produces a variable current-representative signal, and
c) feedback controlling the dc voltage and current in relation to the variable current-representative signal to adjust the dc voltage and current to amplitudes that keep the light intensity produced by the light source substantially constant.
The objects, advantages and other features of the present invention will become more apparent upon reading of the following non-restrictive description of a preferred embodiment thereof, given by way of example only with reference to the accompanying drawings.